1. Technical Field
The present invention relates to a method for manufacturing a flexible printed circuit board, and especially to a method for manufacturing a multilayer flexible printed circuit board having different numbers of layers in different areas.
2. Discussion of Related Art
Flexible printed circuit boards (FPCB) have been widely used in electronic products such as mobile phones, printing heads and hard disks. In these electronic products, some movable parts are advantageously connected to a main body via a flexible printed circuit board. FPCB can assure power supply and signal transmission in such environment due to their excellent flexibility.
FIG. 5F shows a multilayer FPCB structure, which has different number of layers in different areas; in other words, there are a thick area with a number of layers and a thin area with less layers in a same FPCB. The thick area can have a higher circuit density whilst the thin area exhibits higher flexibility.
FIGS. 5A to 5F show a conventional process for manufacturing such type of FPCB. As is shown in FIGS. 5A and 5B, a first copper clad laminate (CCL) 41, a binder layer 45, a second CCL 42 are laminated sequentially. As is shown FIG. 5C, dry films 412, 422 are applied on the first CCL 41 and the second CCL 42 respectively, and then the dry films 412, 422 are exposed and developed. Because there is a cliff-like thickness difference between the first CCL 41 and the second CCL 42, a gap 46 is formed at the “cliff”.
As is shown in FIG. 5D, the first CCL 41 and the second CCL 42 are etched using an etchant and the dry films 412, 422 are removed, the etching step, the etchant can seep into the gap 46 and react with dielectric layers in the first CCL 41 and the second CCL 42. As a result the dielectric layers may peel off from the CCL.
Referring to FIG. 5E, a third CCL 43 and a fourth CCL 44 are laminated with the first CCL 41 and the second CCL 42 respectively. Referring to FIG. 5F, a through hole 47 is formed. The through hole 47 can be made by drilling or by laser ablation. After the through hole 47 is formed, a conductive layer is formed on a surface of the through hole 47 by electroless plating or electroplating. In the plating process the dielectric layer of the second CCL 42 is exposed to a plating solution thereby forming a number of copper lumps thereon. These copper lumps can pierce dry film that applied on the second CCL 42 in the next pattern-forming process, and etchant used for developing the dry film can react with dielectric layer or copper layer of second CCL 42 and cause poor quality product to be formed.
In the aforementioned process for manufacturing multilayer FPCB that has different number of layers in different areas, a fall structure between different CCLs can causes a series of quality problems, therefore there is a desire to develop a process that can solve aforementioned quality problems.